THIS YEAR, THE 50th ANNIVERSARY of the discovery of the
structure of DNA has kindled many debates about the implications of that
knowledge for the human condition. Arguably the most emotionally charged
is the debate over the prospect of human genetic enhancement, or
''designer babies.'' It's only a matter of time, many say, before parents
will improve their children's intelligence and personality by having
suitable genes inserted into them shortly after conception.

A few commentators have welcomed genetic enhancement as the latest step
forward in the age-old struggle to improve human life. But many more are
appalled. They warn that it is a Faustian grab at divine powers that will
never be used wisely by us mortals. They worry that it will spawn the
ultimate form of inequality, a genetic caste system. In his book ''Our
Posthuman Future'' (just released in paperback), the conservative thinker
Francis Fukuyama warns that genetic enhancement will change human nature
itself and corrode the notion of a common humanity that undergirds the
social order. Bill McKibben, writing from the political left, raises
similar concerns in his new jeremiad ''Enough: Staying Human in an
Engineered Age.''

But whether they welcome or decry it, almost everyone agrees that
genetic enhancement is inevitable if research proceeds on its current
course. Genetic enhancement is a major concern of the President's Council
on Bioethics; its chairman, Leon Kass, and several of its members,
including Fukuyama, are outspoken worriers.

As it happens, some kinds of genetic enhancement are already here.
Anyone who has been turned down for a date has been a victim of the human
drive to exert control over half the genes of one's future children. And
it is already possible to test embryos conceived in vitro and select those
that are free of genetic defects such as cystic fibrosis.

But when it comes to direct genetic enhancement-engineering babies so
they will carry genes for desirable traits-there are many reasons to be
skeptical. Not only is genetic enhancement not inevitable, it is not
particularly likely in our lifetimes. This skepticism arises from three
sources: futurology and its limits, the science of behavioral genetics,
and human nature itself.

The history of the future should make us raise an eyebrow whenever the
experts tell us how we will live 10, 20, or 50 years from now. Not long
ago, we were assured that by the turn of the century we would live in
domed cities, commute by jet-pack, and clean our homes with
nuclear-powered vacuum cleaners wielded by robot maids. More recently we
were promised the paperless office, interactive television, the Internet
refrigerator, and the end of bricks-and-mortar retail. It's not just that
these developments have not yet happened. Many of them, like domed cities,
never will happen. Even in mundane cases, technological progress is far
from inexorable. Air travel, for example, is barely faster or more
comfortable today than it was when commercial jets were introduced 50
years ago.

Why are technological predictions usually wrong? Many futurologists
write as if current progress can be extrapolated indefinitely-committing
the fallacy of climbing trees to get to the moon. They routinely
underestimate how much has to go right for a development to change our
lives. It takes more than a single ''eureka!'' It takes a large number of
more boring discoveries, together with the psychological and sociological
imponderables that make people adopt some invention en masse. Who could
have predicted that the videophones of the 1960s would sink like a stone
while the text messaging of the 1990s would become a teenage craze?

Finally, futurologists tend to focus their fantasies on the benefits of
a new technology, whereas actual users weigh both the benefits and the
costs. Do you really want to take the time to install software upgrades on
your refrigerator, or reboot it when it crashes?

Many prognosticators assume that we are currently discovering single
genes for mathematical giftedness, musical talent, athletic prowess, and
the like. The reality is very different. The Achilles heel of genetic
enhancement will be the rarity of single genes with consistent beneficial
effects.

Behavioral genetics has uncovered a paradox. We know that tens of
thousands of genes working together have a large effect on the mind.
Studies show that identical twins (who share all their genes) are more
similar than fraternal twins (who share half of those genes that vary from
person to person), who in turn are more similar than adopted siblings (who
share even fewer of the varying genes). Adoption studies show that
children tend to resemble their biological relatives in personality and
intelligence more than they resemble their adoptive relatives.

But these are the effects of sharing an entire genome, or half of one.
The effects of a single gene are much harder to show. Geneticists
have failed to find single genes that consistently cause schizophrenia,
autism, or manic-depressive disorder, even though these conditions are
substantially heritable. And if we can't find a gene for schizophrenia,
we're even less likely to find one for humor, musical talent, or
likeability, because it's easier to disrupt a complex system with a single
defective part than to improve it by adding a single beneficial one. The
1998 report of a gene that was correlated with a 4-point advantage in IQ
was recently withdrawn because it did not replicate in a larger sample-a
common fate for putative single-gene discoveries.

So don't hold your breath for the literary-creativity gene or the
musical-talent gene. The human brain is not a bag of traits with one gene
for each trait. Neural development is a staggeringly complex process
guided by many genes interacting in feedback loops. The effect of one gene
and the effect of a second gene don't produce the sum of their effects
when they're simultaneously at work. The pattern of expression of
genes (when they are turned on or off by proteins and other signals) is as
important as which genes are present.

Even when genes should be at their most predictable-in identical twins,
who share all their genes, and hence all the interactions among their
genes-there are no foregone conclusions about anyone's traits or behavior.
Identical twins reared together, who share not only their genes but most
of their environment, are imperfectly correlated in personality measures
like extroversion and neuroticism. The correlations, to be sure, are much
larger than those for fraternal twins or unrelated people, but they are
seldom greater than .5. This tells us there is an enormous role for chance
in the development of a human being.

It gets worse. Most genes have multiple effects, and evolution selects
those genes that achieve the best compromise between positive and negative
impacts. Take the most famous case of genetic enhancement on record: the
mice that were given extra copies of the NMDA receptor, which is critical
to learning and memory. These poster mice did learn mazes more quickly-but
they also turned out to be hypersensitive to pain. Closer to home, there
is a gene in humans that may be correlated with a 10-point boost in IQ.
But it is also associated with a 10-percent chance of developing torsion
dystonia, which can confine the sufferer to a wheelchair with
uncontrollable muscle spasms.

So even if genetic enhancement could work in principle, the problem is
how to get there from here. How can scientists try out different genes to
enhance the minds of babies given that many of those genes could have
terrible side effects?

Genetic enhancement faces another problem: Most traits are desirable at
intermediate values. Wallis Simpson said that you can't be too rich or too
thin, but other traits don't work that way. Take aggressiveness. Parents
don't want their children to be punching bags or doormats, but they don't
want Attila the Hun, either. Most want their children to face life with
confidence rather than sitting at home cowering in fear, but they don't
want a reckless daredevil out of ''Jackass.'' So even if a gene had some
consistent effect, whether the effect was desirable would depend on what
the other tens of thousands of genes in that child are doing.

The third obstacle to re-engineering human nature comes from human
nature itself. We are often told that it's only human for parents to give
their children every possible advantage. Stereotypical yuppies who play
Mozart to their pregnant bellies and bombard their newborns with flash
cards would stop at nothing, it is said, to give their children the
ultimate head start in life.

But while parents may have a strong desire to help their children, they
have an even stronger desire not to hurt their children. Playing
Mozart may not make a fetus smarter, but it probably won't make it
stupider or harm it in other ways. Not so for genetic enhancement. It is
not obvious that even the most overinvested parent would accept a small
risk of retardation in exchange for a moderate chance of improvement.

Our intuitions about naturalness and contamination are another speed
bump on the road to genetically engineered babies. In all cultures, it's
widely believed that living things have essences which give them certain
powers and which can be contaminated by pollutants. These intuitions have
been powerful impediments to the acceptance of various technologies. Many
people are repelled by genetically modified foods even though those foods
have never been shown to be unsafe or harmful to the environment. If
people are repulsed by genetically modified soybeans, would they really
welcome genetically modified children?

Finally, anyone who has undergone in-vitro fertilization knows that
it's a decidedly unpleasant procedure, especially in comparison to sex.
Infertile couples may choose it as a last resort, and some kooks may
choose it to have a child born under a certain astrological sign or for
other frivolous reasons. But people who have the choice generally prefer
to conceive their children the old-fashioned way.

It is misleading, then, to assume that parents will soon face the
question, ''Would you opt for a procedure that would give you a happier
and more talented child?'' When it's put like that, who would say no? The
real question will be, ''Would you opt for a traumatic and expensive
procedure that might give you a slightly happier and more talented child,
or might give you a less happy, less talented child, or might give you a
deformed child, and probably would make no difference?'' For genetic
enhancement to ''change human nature,'' not just a few but billions
of people would have to answer yes.

My point is not that genetic enhancement is impossible, just that it is
far from inevitable. And that has implications. Some bioethicists have
called for impeding or criminalizing certain kinds of research in genetics
and reproductive medicine, despite their promise of improvement in health
and happiness. That is because the research, they say, will inevitably
lead to designer babies. If genetic enhancement really were just around
the corner, these proposals would have to be taken seriously. But if the
prospect is very much in doubt, we can deal with the ethical conundrums if
and when they arise. Rather than decrying our posthuman future, thinkers
should acknowledge the frailty of technological predictions. They should
base policy recommendations on likelihoods rather than fantasies.

S teven Pinker is a professor of brain and cognitive sciences
at MIT and the author most recently of ''The Blank Slate: The Modern
Denial of Human Nature.''